feat: export default branch for orgs repos endpoint

[Barbapapazes]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Hello,

In the same way the default branch is exported for a single repo, having this info for all repos from an orgs could be useful.

related to UnJS Relations where I need to know the default branch when I fetch every repos from an organization

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[Barbapapazes]

Hey @pi0, do you this could be merged? 👀